- Operational definitions: Definitions based entirely on particular observations.
- The earliest concepts of acids were operational definitions, based on observations such as:
- Sourness when tasted.
- Ability to change the colours of indicators.
- Conceptual definitions: Definitions that attempt to explain observations.
- As the field of chemistry developed, concepts of acids began to expand into conceptual definitions.
|Year||Individual||Acid theory||Evidence / Details|
|Acids were substances that contained oxygen|
|1810||Humphry Davy||Acids were substances that contained hydrogen that could be replaced totally or partly by metals||Based on the observation that when acids reacted with metals, the metal appeared to replace the hydrogen in the acid, causing hydrogen gas to be evolved|
|1884||Svante Arrhenius||Acids were substances that ionised in solution to produce hydrogen ions|